On Tuesday evening, Senate Judiciary Committee Chairman Chuck Grassley released a memo from McCullough that adds new detail to the previous finding by the intelligence IG and the State Department IG that at least four of Clinton's emails contained classified information.
There are at least two emails that include information classified up to "top secret" — which is the highest classification category — while two others are under review at the State Department for a determination on their classification, the memo states.
The finding is certain to inflame GOP political attacks against Clinton at a time of already-intense scrutiny of the Democratic White House front-runner's unusual private email arrangement.
The memo to top lawmakers on several committees responds to inquiries about a July memo from McCullough about a limited review that turned up classified information in four emails, although they did not have classification markings or "dissemination controls" at the time.
The State Department is not currently endorsing McCullough's claim that two of Clinton's messages contained "top secret" information.
"The Intelligence Community has recommended that portions of two of the four emails identified by the Intelligence Community's Inspector General should be upgraded to the Top Secret level. Department employees circulated these emails on unclassified systems in 2009 and 2011 and ultimately some were forwarded to Secretary Clinton. They were not marked as classified," State Department spokesman John Kirby said Tuesday night.
Kirby said that State will ensure the information is protected while his agency works with the Director of National Intelligence to "resolve whether, in fact, this material is actually classified."
His comments underscore the ongoing lack of consensus among officials about classification.
A senior State Department official said it's common that in cases of "parallel" information gathering from sensitive and publicly available sources, "the source from which the information is derived is critical in determining classification."
"It is common for State Department employees to learn information from open sources, including press reports, that may also be independently learned through entirely separate means within the Intelligence Community," the State Department official said.
Clinton's campaign has said she did not send or receive emails that were marked classified at the time.
Grassley, in a statement, cheered the new details about classification provided by McCullough to lawmakers.
"I appreciate the intelligence community inspector general providing more information in response to the questions that many members of Congress and the public have regarding the classified emails that were on former Secretary of State Clinton's private server and on a thumb drive with her private attorney. This information revealed by the inspector general makes it even more important that the FBI and the State Department secure these documents," Grassley said in a statement made before the campaign's announcement.